System for generating intense pulses of microwave power using traveling wave acceleration means



Jan. 13, 1970 A. s. DENHOLM SYSTEM FOR GENERATING INTENSE PULSES OF MICROWAVE Filed Nov. 14,

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EN HOLM ATTORNEY Jan. 13, 1970 A5. DENHOLM SYSTEM FOR GENERATING INTENSE PULSES OF MICROWAVE POWER USING TRAVELING WAVE ACCELERATION MEANS 14, 1966 2 Sheets-Sheet 2 Filed Nov.

ALEC. s DENHL BY MA ATTORNEY INVENTOR So E mm 5&3

United States Patent M SYSTEM FOR GENERATING INTENSE PULSES 0F MICROWAVE POWER USING TRAVELING WAVE ACCELERATION MEANS Alec S. Denholm, Lexington, Mass., assignor to Ion Physics Corporation, Burlington, Mass., a corporation of Delaware Filed Nov. 14, 1966, Ser. No. 593,816 Int. Cl. H01j 25/02, 25/10 US. Cl. 315-5 15 Claims ABSTRACT OF THE DISCLOSURE A microwave power device having an electron beam source which emits electrons through a velocity modulating resonant cavity, wherein the velocity of the introduced beam is modulated to bunch the beam passing therethrough, and into a drift tube through which the bunched beam passes and around which high voltage is deployed, the high voltage being sustained only for the duration of time required for the bunched electrons to pass through the drift tube by establishing a traveling wave which passes and repasses between the drift tube and a. sleeve surrounding the drift tube, the improved high voltage causing the beam to be accelerated, as it passes out of the drift tube into a region affected by the high voltage, through a second resident cavity, with a power level in the order of megawatts, where the greatly amplified signal is extracted from the velocity modulated, accelerated beam, into a collector where the spent electrons are absorbed.

This invention relates generally to microwave system and more particularly to a novel, fast pulse, high power, megavolt microwave systems.

Recent studies have shown that the microwave breakdown field in the atmosphere, for example, at a transmitting antenna on the earths surface is very high for nanosecond pulse lengths. This is of great importance to the design of equipment for the discrimination of small radar targets at a distance, since adequate discrimination requires the radiation of very high power nanosecond microwave pulses of short duration. Unfortunately, the short pulse field strength cannot be fully utilized because of limitations in existing high power tubes e.g., klystrons, and it is necessary to look for either a microwave generation concept which does not depend on a high power output tube, or a new concept in high power tube design.

The microwave power output of the most powerful tubes, namely klystrons, can approximately be equated with the product of the electron current and the electron beam energy in the tube; i.e., the voltage through which they are accelerated. Electron currents are limited to values of the order 100 amperes because of the maximum available emission from thermionic cathodes, and electron energies are limited to about 300 kv. because of voltage breakdown problems. The klystron has to withstand continuously the maximum voltage corresponding to the beam energy. The available power from a conventional klystron is then seen to be limited to the order 3X10 watts. The present invention, described below, can either retain thermionic cathode limitations and increase acceleration voltage by one order of magnitude so that power levels are 3x10 watts or greater, or it can substitute afield emission cathode for the thermionic cathode and increase the current level by about two orders of magnitude so that power levels of 3l l0"-3 l0 watts are possible.

Broadly speaking the present invention accomplishes this result in a tube which comprises, a field or thermionic emission electrode for supplying a beam of electrons to 3,489,943 Patented Jan. 13, 1970 the body, velocity modulating means for bunching the electrons, acceleration means for accelerating the electrons, a microwave cavity for extracting the signal from the apparatus and a collector for the spent electrons.

The invention and all its features and advantages will be more fully appreciated from the following description taken in conjunction with the accompanying drawings in which;

FIGURE 1 shows a microwave tube embodying the features of the invention,

FIGURE 2 illustrates a different embodiment of the invention, and

FIGURE 3 illustrates additional embodiments of the invention.

The discussion will refer to the field emission electron approach because of its novelty, but it is understood that the source of bunched electrons could be a state of the art klystron bunching gun with a thermionic cathode.

Referring to the figures the present invention comprises a source of intermediate energy electrons 10 forming one end of the apparatus and a collector electrode structure 13 forming the other end. The intermediate portion of the tube comprises a housing 11 which contains radio frequency interaction means together with a means of electron acceleration to the ultimate energy.

The electron source structure 10 is made up of three distinct but interacting parts, a coaxial capacitor (heregas filled), a gas discharge switch and a field emission cathode. The capacitor comprises a grounded hollow metal electrode 14 and an inner electrode 17 which is insulatively maintained in a coaxial spaced relationship with the outer electrode 14 by suitable means (not shown). Electrode 14 is sealed at one end with a cap 15 and at the other end with a ground plate 18. This outer electrode must be capable of withstanding an internal filling of high pressure insulating gas, in this case 400 p.s.i. A high voltage feed through bushing 16 passes through cap 15 and connects the inner electrode 17 to a power supply 19. In the preferred embodiment the power supply 19 is an insulating core power supply that resonantly charges the electrode 17 through an isolating inductance. Located in line with the center end of the central electrode 17 but separated therefrom by gas discharge switch 25 is a field emission cathode structure 20 which is comprised of a cap plate 21, a field emission cathode 22, mounted thereon, and an insulating, hollow, evacuated. cylindrical standoff 23 which surrounds the cathode 22 and extends between the plate 21 and the ground plane 18. At the juncture of standoff 23 and ground plane 18 there is provided an orifice 12, which connects the interior of housing 11 with the interior of standoff 23. Preferably, the standoff 23 is capacitively graded, that is, is formed of dielectric rings alternated with metal ring electrodes and orifice 12 is sealed with a thin metallic window 24 which acts as an anode for the cathode 22.

The dimensions of the structure 10 depends on the desired intensity and duration of the electron beam which is to be extracted from cathode 22. Typically the inner electrode is about 3 meters long (for a 20 nanosecond pulse length) and has a capacitance of 280 pf. The switch 25 is a (typical 2.5.cm.) gap (but depending on desired voltage and gas filling) separating the tip of electrode 17 from the cap 21, while the length of standoff 23 is determined by the beam voltage and typically is 4 inches per megavolt. For a 500 kev. electron injection energy, gap 25 might be 2.5 cm. and the length of standoff 23 about 3 inches.

It should be noted at this time that the capacitor section of the apparatus could be filled with either a solid or liquid dielectric instead of the described gas and could be either a lumped or distributed transmission line.

The intermediate portion 11 comprises a microwave buncher 26 and a catcher 27 separated by an elongated Faraday cage or drift tube 28 and a second insulating standoff 29. The buncher 26 which velocity-modulates the electrons is shown as a cavity resonator 30 which bridges an interaction gap 31 between the ground plane 18 and a flange 32 extending between one end of drift tube 28 and the external wall 34 of intermediate section 11. It is of course to be understood that for the sake of simplicity in the drawing that only one cavity resonator and interaction gap is shown and that a plurality of these could be provided. The resonant cavity has a radio frequency standing wave oscillation established therein by means of a probe 32 which feeds the wave into the cavity from a suitable source (not shown).

The other end of drift tube 28 is maintained in an insulated spaced relationship, by standoif 29, with the interior of a second microwave cavity 27 which acts as a means of coupling the RF frequency from the apparatus via probe 35. Thus this second microwave cavity serves as a microwave catcher.

Surrounding the drift tube 28 and coaxial therewith is a sleeve electrode 36 having a switching location 37 thereon. This sleeve 36 is connected to a high voltage source 38 by a feed through bushing 39 which extends through the wall 34. The wall 34 is made to be pressurized so that the intermediate portion 11 may also be pressurized. The interior of the drift tube 28 is however either maintained at a vacuum or a very low gas pressure.

The described apparatus operates when the voltage is permitted to build up to the selected level on electrode 17. For purposes of this explanation it will be assumed that th voltage build up on electrode 17 is 1 megavolt and that this electrode has a discharge length of 3 meters. When this voltage level is reached a gas discharge is initiated between electrode 17 and cap plate 21 across switch 25. If desired this discharge is initiated by an axial triggering electrode, an external stimulus, such as a laser beam, or by overvolting.

When the discharge occurs the stored energy is transferred from the coaxial line to cap 21 and thus to the field emission cathode 22. Instantaneously potential appears between the cathode 22 and the grounded window 24 causing a beam of electrons to be extracted from cathode 22 by means of the field emission process.

If the coaxial line has an impedance where R is the radius of 14 and is the radius of 17, then the impedance of the field emission diode 22/24 can be designed so that it has the value 409 at half the voltage to which 17 is charged (Z then the electron beam accelerating voltage is half the charge voltage on 17, i.e. the line is matched, and a square pulse is produced. Taking some practical numbers with Z =40Q, the electron beam current would be 12.5 ka. at 500 kev. If the line is 3 meters long this pulse will have a duration of approximately nanoseconds.

The electron beam travels towards the orifice 12 and passes therethrough into the interaction space. This orifice 12 may be open or covered with a screen or foil thin enough to transmit the electrons. A foil 24 is used if it is desired to maintain different vacuum conditions in the interaction gap 31 and in housing 23.

As is Well known in the microwave tube art radio frequency oscillations introduced into resonant cavities will produce varying electrostatic fields across the interaction gap to modulate the velocity of the electron passing through the gap. Such modulation results in bunching of the electrons into small bundles. After emerging from the interaction gap the bundles enter the drift tube 28 and travel through it towards collector 13.

If desired a conventional magnetic system (not shown) may be provided around the drift tube sections to prevent the electron beam from spreading excessively. Possibly this magnetic control can be dispensed with in the present invention for it has been discovered and discussed in the literature that by permitting a small amount of residual ionizable gas at a pressure between 10* and 10- torr to remain in the tube body 28 that a continuously self constricting electron beam can be created. This is possible in the present invention because of the brief, intense beam currents provided by the present invention.

Such self constriction provides many benefits whereby the overall complexity of the tube body 28 and associated equipment may be reduced. This feature is highly desirable.

Approximately 20 nanoseconds after the discharge of line 17 and the introduction of the electron beam into the interaction gap the line 17 is totally discharged and the beam extinguishes itself because the voltage on cathode 22 falls below the level to sustain the beam. As the first of the electrons in the drift tube 28 approaches the end adjacent to 29 a potential, say on the order of 4 megavolts, previously built up on sleeve 36, is permitted to discharge across projection 37 to tube 28. This projection 37 serves as a switch and may be activated as was switch Instantly the open end of drift tube 28 under projection 37 rises to a potential approaching 4 megavolts. The first of the electron bunches reach the end of the long drift tube 28 and begin to enter the standoff 29. These electrons thus experience the high field now existing between tube 28 and window or aperture 40, positioned at the entry to catcher 27, and are accelerated. Because of beam loading the potential drops from 4 mv. to about a 3 mv. depending on the geometrics chosen and it is to that level the quanta are accelerated.

The discharge of sleeve 36 to tube 28 establishes traveling electric field between the sleeve and the tube which begins to move towards the end of the tube grounded at wall 32. When this field arrives at wall 32 it is canceled to zero field by the reflected wave, but the accelerator section 29 does not drop in potential until the wave reflected from 32 arrives at 29, by which time all the electron bunches have been accelerated. If 36 is 3 meters long, the wave initiated at 37 takes 20 nanoseconds to be reflected and return to 37 and reduce the accelerating field across 29 to zero.

Because standoff 29 is built to withstand the applied voltage of 4 mv. no discharge takes place across standoff 29. It has been found that standoff 29 can support 3 mv./ft. It is of course understood that this standoff 29 is also capacitivelygraded as was standoff 23.

During this period of 20 nanoseconds while the open end of tube 28 remains at the 3 mv. level the electron bundles emerging from the open end of tube 28 becomes accelerated due to the electric field existing between the open end of tube 28 and window 40 which seals the interior of tube 29 from the interior of catcher 27. It may not be necessary for 40 to be a window (thin foil), in which case it is simply an aperture. The accelerated electrons are driven by this field through the window 40 and catcher 27 to collector 13. By carefully establishing the times involved the field at the open end of tube 28 will not collapse until after the last bundle has emerged and has been accelerated.

During passage of the accelerated beam through the microwave catcher the now amplified signal is extracted therefrom by lead probe 35. After passing through the catcher the electrons proceed to collector 13 where the residual energy of the beam is dissipated. Because these electrons can have an energy of at least 3 million volts it is desirable that collector 13 be composed of low Z material to reduce the production of X-rays by the electrons striking the collector. It is for this reason that collector 13 must be locally shielded.

Further, the discharging line in 11 can have more than one form. It can operate on the Blumlein principle as shown in FIGURE 2. Here the switching is between 41 and ground, which may be technically easier than between 37 and 28 (FIG. 1). The traveling field injected at 41 by the grounding action of the switch travels to 29 and is doubled to give the Blumlein action and thus voltage across the accelerator section 29. Pulse duration of the potential across 29 is determined by the length of the Blumlein.

Also the pulse generator 17a could comprise an electr static belt generator whose terminal forms the central conductor 17.

It should now be obvious that the given dimension, currents, voltage values, etc. can be changed to any desired levels as noted and therefore it is desired that the invention be limited only by the appended claims.

What is claimed is:

1. An electro-magnetic wave generator comprising, means for producing a beam of electrons, means for velocity-modulating said beam to produce a bunched beam of electrons, a hollow field free tube electrically grounded at one end and isolated from ground at the other end through which said bunched beam passes, means accelerating said bunched beam to megavolt potentials, said accelerating means comprising means surrounding said drift tube and means for impressing a traveling megavolt electromagnetic wave between said surrounding means and said drift tube to produce a field between ground and said isolated end at megavolt potential, a radio frequency interaction means for coupling energy from said accelerated beam and means for collecting said beam.

2. The device of claim 1 wherein said means for producing comprises a pulse forming system consisting of a power supply electrically connected to an insulated transmission line coupled by a gas discharge switch to a field emission cathode contained in an evacuated envelope, said cathode being electrically isolated from said velocitymodulating means by a vacuum gap and a grounded anode.

3. The device of claim 1 wherein said drift tube contains means for causing self constriction of said beam while said beam is passing through said drift tube, and said means for causing self-constricting of said beam comprises an ionizable gas maintained in said drift tube at a pressure between and 10 torr.

4. A microwave electron discharge device comprising a housing containing means for generating a kilo amper pulse of megavolt electrons, said generating means comprising a pulse generator and a field emission cathode, means for acting on said pulse to amplify a microwave signal, said acting means comprising a low Z collector, a plurality of alternating electrodes cavity resonators, means for establishing a field in at least one of said cavity resonators to velocity modulate electrons passing therethrough, regenerative coupling means among said cavity resonators to create sustained oscillations in said device, and means for accelerating said pulse after said pulse is modulated, said accelerating means comprising a drift tube surrounded by a sleeve to form an open-ended co-axial line coupled to means for establishing a traveling wave in said line, one of said cavity resonators extracting energy from said accelerated pulse, and means for separating said acting means from said generating means, said separating means comprising a grounded anode disposed between said cathode and said acting means.

5. The device of claim 4 wherein said means for generating comprises an electrostatic belt generator whose terminal forms the central conductor of a gas insulated capacitor coupled through a gas discharge switch to the field emission cathode.

6. The device of claim 1 wherein said means for producing comprises a thermionic cathode.

7. The device of claim 1 wherein said means for producing comprises a field emission cathode.

8. The device of claim 1 wherein said means for impressing comprises a megavolt potential power source coupled to said means surrounding said drift tube and means for transferring a potential built up on said means surrounding said drift tube to said drift tube.

9. A high power microwave amplifying device for amplifiyng microwave signals to power levels in the order of 10 watts comprising means for producing a multikilo ampere current of electrons, a low Z collector, radio frequency interaction structures and an electron acceleration means, said producing means comprising means for storing energy and means for delivering said energy in the form of a pulsed kiloampere electron beam through one of said structures, said beam having a pulse length of nanoseconds, one of said structures comprising a drift space and a resonant cavity with electric fields established in said cavity to bunch the electrons in said beam by alternately increasing and decreasing their velocities, said acceleration means comprising a drift tube surrounded by a sleeve to form an open-end co-axial line and coupled to means for establishing a traveling wave between said drift tube and said sleeve and imposing an electric field between said drift tube and a ground plane positioned between the drift tube and one of said structures comprising a drift space and a resonant cavity for extracting energy from the bunched, accelerated electrons.

10. The device of claim 1 wherein said drift tube and said surrounding means are elongated and have effective electrical lengths for said traveling wave equal to the time required for said bunched beam to .pass the isolated end of said tube such that the impressed potential creating said accelerating field exists on said isolated end for a time period whose duration is equal to the time period that said velocity modulated beam takes to pass the isolated end of said drift tube.

11. The device of claim 1 wherein said tube and said surrounding means are elongated and have effective electrical lengths for said traveling wave greater than the time required for said bunched electron beam to pass the isolated end of said tube such that the impressed potential creating said accelerating field exists on said isolated end for a time period whose duration is longer than the time period that said beam takes to pass the isolated end of said tube.

12. The apparatus of claim 1 wherein said impressing means comprises means for establishing a charge on said surrounding means and switching means for discharging said charge to said drift tube.

13. The apparatus of claim 12 wherein said surrounding means is coaxial with said drift tube.

14. The apparatus of claim 12 wherein said surrounding means is a lumped constant line.

15. The apparatus of claim 12 wherein said surrounding means is a Blumlein.

References Cited UNITED STATES PATENTS 1/1949 Horsley 3l55.34 9/1966 El-Hefni 3155.41

, OTHER REFERENCES HERMAN KARL SAALBACH, Primary Examiner S. CHATMON, JR., Assistant Examiner U.S. C1. X.R.

s13 211, 336; 31s-s.s9, 5.41; 328-233 

